Fix some errcheck errors (#9811)

* Throw away result of multierror.Append

When given a *multierror.Error, it is mutated, therefore the return
value is not needed.

* Simplify MergeMultierrorWarnings, use StringBuilder

* Hash.Write() never returns an error

* Remove error that was always nil

* Remove error from Resources.Add signature

When this was originally written it could return an error, but that was
refactored away, and callers of it as of today never handle the error.

* Throw away results of io.Copy during Bridge

* Handle errors when computing node class in test

client/devicemanager/manager.go

@@ -283,13 +283,13 @@ func (m *manager) cleanupStalePlugins() error {
 	for name, c := range s.ReattachConfigs {
 		rc, err := pstructs.ReattachConfigToGoPlugin(c)
 		if err != nil {
-			multierror.Append(&mErr, fmt.Errorf("failed to convert reattach config: %v", err))
+			_ = multierror.Append(&mErr, fmt.Errorf("failed to convert reattach config: %v", err))
 			continue
 		}
 
 		instance, err := m.loader.Reattach(name, base.PluginTypeDevice, rc)
 		if err != nil {
-			multierror.Append(&mErr, fmt.Errorf("failed to reattach to plugin %q: %v", name, err))
+			_ = multierror.Append(&mErr, fmt.Errorf("failed to reattach to plugin %q: %v", name, err))
 			continue
 		}